The use of ultraviolet (UV) radiation for phototherapy is well established. In fact, UV therapy is now involved in the treatment of more than 40 types of skin diseases and disorders, such as psoriasis, vitiligo, and eczema. Phototherapy studies of UVB wavelengths between 260 nm and 320 nm found that a narrow-band UV emission centered at approximately 312 nm is most effective for phototherapy while at the same time limiting undesirable erythemal effects. Since the skin's erythemal (or sunburning sensitivity) is at its maximum at about 297 nm, a narrow-band emission at about 312 nm allows a patient to have longer treatment times before an erythemal response appears.
One known UV-emitting phosphor is a magnesium pentaborate sensitized with cerium, e.g., (Gd0.45,Y0.5)MgB5O10:Ce0.05, which is described in U.S. Pat. No. 4,319,161. The method of producing Ce-activated magnesium pentaborate phosphor involves dry mixing boric acid and oxides of Gd, Y, Ce, and Mg, and then subjecting the mixture to three firings in a weakly reducing atmosphere. Later in U.S. Pat. No. 6,007,741, Hunt et al. described an improved process to prepare the (Gd0.45,Y0.5)MgB5O10:Ce0.05 phosphor by milling the reactants in a saturated aqueous solution of magnesia and boric acid prior to firing three times. The later method was developed to increase the homogeneity of the fired phosphor cake and reduce the tendency of the fired cake to stick to the firing boats. Even more recently in U.S. Pat. No. 7,288,215, Fan et al. describe an improved method of making (Gd0.45,Y0.49)MgB5O10:Ce0.05,Pr0.01 UV-emitting phosphors that involves mixing boric acid, magnesia and a previously prepared co-precipitate of (Gd,Y,Ce,Pr) oxide and then subjecting the mixture to two firing steps in a weakly reducing atmosphere. Although the use of the mixed oxide co-precipitate improves homogeneity and yields a phosphor having a higher brightness, this process still uses excess boric acid as the boron source.
For all the processes mentioned above, the material after the first firing is commonly inhomogeneous due to melting and separation of the boric acid raw material. The fired cake subsequently needs to be washed, milled, re-blended with additional boric acid for the second or third firing step. This repeated grinding and firing is labor intensive and the large quantity of boric acid can contaminate the furnace.
Unlike the above methods, U.S. Pat. Nos. 4,719,033 and 5,068,055 describe a single-step firing process for making europium-activated strontium tetraborate, SrB4O7:Eu, a UVA-emitting phosphor. The process involves adding a SrCO3/Eu2O3 mixture to an H3BO3 slurry at >90° C. to form a (Sr,Eu)B6O10.5H2O precipitate along with excess SrCO3/Eu2O3 in a 2:1 ratio. The precipitate is then fired to yield the SrB4O7:Eu phosphor. No boric acid is used in the firing step. The hydrated precipitate is fired after drying without adding additional compounds.
It would be advantageous to have a firing process for making the UV-emitting magnesium pentaborate phosphors which avoids the use of excess boric acid and does not require multiple firing steps.